This invention relates to electrical load protection devices, and is directed particularly to electric motor protection devices or line monitors offering protection against low or high energizing voltages and momentary voltage interruptions which might otherwise damage a motor applied as the line load. Such load detectors or line monitors are particularly advantageous when applied to the electrical energization source of refrigeration compressor motors, such motors being particularly susceptible to damage caused by low or high voltages, or momentary power interruption. With excessively high or low energization voltage, the motor will draw abnormally high current, subjecting it to accelerated failure or burn-out. With momentary power interruption, existing forces of compression within the unit cause such excessively high starting currents as could result in almost immediate burn-out. This is particularly true when the compressor unit back pressure load is high enough to result in a locked rotor upon attempted automatic re-start after a very brief power interruption.
Such devices heretofore devised for protection against early failure due to overheating or burn-out include thermal interrupter switches and electronically controlled line monitors of one type or another in which low energizing voltage or momentary voltage interruptions are sensed for activation or deactivation of a switching relay in series with the energizing voltage line. Such protective devices are deficient in various respects, principally in that they are too slow-acting to prevent some damaging heat build-up, and in that they are not sufficiently responsive to over-voltage as an insidious long or short term damaging condition.
It is, accordingly, the principle object of this invention to provide a novel and improved line monitor or protective device for electric motors, particularly but not exclusively for refrigeration equipment compressor motors, that obviates the deficiencies such devices heretofore devised.
A more particular object of the invention is to provide an electrical load protection device of the character described which protects against over-voltage as well as under-voltage within narrow and precise tolerance thresholds.
Another object of the invention is to provide an electrical load protection device of the character described that provides for selection to a nominal line voltage determining the actual pickup and drop-out of the threshold voltages.
Another object of the invention is to provide an electrical load protection device of the above nature wherein, upon activation resulting from any detected over-voltage, under-voltage or voltage interruption condition, power is cut off for a time sufficient for cooling of the motor to a degree that insures against any liklihood of damage upon the automatic switching on again of the energizing voltage.